A fan motor is mounted in various electronic devices such as server, laptop, desktop computer, portable computer, game device, projector, and VR (Virtual Reality) etc. In addition, a fan motor is also mounted in refrigerator, automobile, industrial machine, or the like.
In a platform having a processor such as CPU (Central Processing Unit) or MPU (Micro-Processing Unit) mounted thereon, the number of revolutions of a fan motor can be controlled in accordance with an instruction from the processor. Specifically, a control signal indicating the target number of revolutions is provided to a fan motor driver from the processor. The fan motor driver generates a PWM (pulse width modulation) signal having a duty ratio corresponding to the control signal so as to drive the fan motor.
In addition, in order to improve the quietness, PWM (Pulse Width Modulation) soft switching is performed by slowly switching the coil current before and after switching of the output phase.
FIG. 1A and FIG. 1B are diagrams showing a state transition of an H-bridge circuit during PWM driving. In the conventional PWM driving, in a state where an output (OUT2) is fixed as a low level (low), another output (OUT1) is switched in accordance with the PWM signal. In a section where the PWM signal is at high level, as shown in FIG. 1A, the transistors MH1 and ML2 are turned on, and the coil current ICOIL flows through the first path including the transistor MH1, a motor coil L, and the transistor ML2. In a section where the PWM signal is at low level, as shown in FIG. 1B, the transistors ML1 and ML2 are turned on, and the coil current ICOIL flows through the second path including the transistor ML1, the motor coil L, and the transistor ML2.
FIG. 2 is a waveform diagram illustrating the PWM soft switching. Time to is the current zero crossing point at which the direction of the coil current is reversed. A soft switching duration Tss is set in such a manner as to include the current zero crossing point. As shown in FIG. 1A and FIG. 1B, the output voltage VOUT1 during the period before the current zero crossing point to in the soft switching duration Tss is pulse width modulated, and its duty ratio changes slowly with time. The output voltage VOUT2 during the period after the current zero crossing point to is pulse width modulated. Accordingly, the coil current ICOIL can be slowly changed, and noise can be reduced.